1. Field of the Invention
The present invention concerns a device for drawing down an optical fiber preform, and more particularly a device of this kind in which the vitreous material preform has a feed rod at one end. This fiber drawing feed rod is usually also made from a vitreous material, of lesser quality than that constituting the preform. It is used to locate the preform in the fiber drawing device, to avoid damaging the preform itself.
2. Related Art
Generally speaking, a fiber drawing device of this kind includes:
means for holding the preform substantially vertical by means of the feed rod, the lower end of the preform being free and the upper end being joined to the feed rod, PA1 means for displacing the preform in a substantially vertically downwards direction so that its lower end enters and advances in a chamber of the fiber drawing device having for this purpose an opening in its upper part, the upper part of the chamber being isolated from the external environment when the preform is present in the opening, and PA1 heating means for heating the chamber to a temperature at least equal to the drawing temperature of the vitreous material constituting the preform, in such manner as to draw down the preform as it moves in the chamber. PA1 means for holding said preform substantially vertical by means of said feed rod so that said preform installed in said device has a free lower end and an upper end joined to said feed rod, PA1 means for displacing said preform in a substantially vertically downwards direction so that its lower end enters and advances in a chamber having for this purpose an opening in its upper part, the upper part of said chamber being isolated from the external environment when said preform is present in said opening, and PA1 heating means for heating said chamber to a temperature at least equal to the drawing temperature of said vitreous material, in such manner as to draw down said preform as it moves in said chamber,
In such devices, it is very important to prevent the external environment from disturbing the interior of the fiber drawing chamber. Disturbances, such as thermal disturbances due to the fact that the environment outside the chamber is at a much lower temperature than the fiber drawing temperature (which is typically in the order of 2 000.degree. C.), can lead to variations in the diameter of the optical fiber produced. Such disturbances can also damage the heating means of the fiber drawing device should air enter via defective seals.
Protecting the interior of the fiber drawing chamber from the external environment is therefore particularly important in the upper part of the chamber, through which the preform enters. The lower part of the chamber, through which the optical fiber leaves, comprises a small diameter calibrated orifice providing a seal by controlled pressurization inside the fiber drawing chamber.
A first solution known in itself to the problem of protecting the upper part of the fiber drawing chamber against disturbance by the external environment is to provide a seal of constant diameter at the opening in the upper part of the chamber.
A seal of this kind is obviously not satisfactory if the fiber drawing feed rod has a diameter less than that of the preform. At the end of drawing, the part of the preform remaining to be drawn is very short and it is therefore the feed rod that moves forward in the opening in the upper part of the fiber drawing chamber. In this case, the fiber drawing chamber can no longer be sealed from the external environment, with all the harmful consequences mentioned above.
Consideration might be given to using a fiber drawing feed rod having the same diameter as the preform, so that the seal at the opening in the upper part of the fiber drawing chamber continues to be effective when the feed rod enters the opening. However, this solution is not satisfactory for various reasons.
Firstly, the advantage of the feed rod of smaller diameter than the preform mentioned above is that it is used to locate the preform during its fabrication, and in particular during the plasma coating step. To use a feed rod fiber drawing having the same diameter as the preform, it is necessary to saw off the original feed rod and to weld on a new feed rod, but these are additional operations which reduce the yield of the optical fiber fabrication process. Moreover, these operations entail the risk of polluting the surface of the preform and therefore of causing the latter to break.
Furthermore, the use of a feed rod the same diameter as the preform is costly since a feed rod of this kind uses more material than a feed rod of smaller diameter.
Finally, because the diameter of the preform is not exactly the same from one preform to another, for each preform it is necessary to measure its diameter and then to machine a feed rod of the same diameter, which is obviously very costly.
Another solution to the problem of maintaining the seal in the upper part of the fiber drawing chamber that has already been envisaged consists in providing at the level of this upper part an enclosure having a lower part in contact with the upper part of the fiber drawing chamber and an upper part provided with a seal having the same diameter as the feed rod, i.e. a smaller diameter than the preform.
The problem with this solution is that, preforms currently being relatively long, in the order of 80 cm to 1 m, and the drawing temperature being reached in the fiber drawing chamber at a distance of about 25 cm below the opening in its upper part, it is necessary to use an enclosure of relatively great height, at least equal to the length of the preform, in order to maintain the seal throughout drawing. It is also necessary to use a feed rod at least as long as the enclosure, so that the preform can be located and moved throughout the drawing operation.
This increases the size of the fiber drawing device. Moreover, the enclosure is itself costly. Finally, because of the total length of the fiber drawing feed rod/preform combination, vibrations of the combination are observed during drawing which lead to variations in the diameter of the optical fiber produced and therefore deterioration of its performance.